Technical Abstract:
Atmospheric transport and deposition constitutes the major mechanism for dispersal of certain rust fungi. Trap nurseries, currently used to follow seasonal movements of rusts, are labor intensive and reflective of past events. An early warning system is needed. Puccinia graminis was used as a model rust system to test whether real-time PCR can detect rust spores in rain samples. Weekly precipitation samples from 19 National Atmospheric Deposition Program (NADP) sites in the "Puccinia Pathway" were used. The assay utilizes rust specific primers and a P. graminis specific TaqMan probe nested within a general primer pair within the ITS1 region of the rDNA. Lower limits of the assay are roughly 1-10 spores per sample based on spiking experiments. P. graminis spores were detected by June 1, 2004, in Minnesota and North Dakota, roughly one month prior to the first field observations in Minnesota. Frequency of detection increased through June, was weekly in July and August, and decreased by September. This study demonstrates that use of a PCR assay to detect spores in rain samples collected at geographically dispersed NADP sites can serve as a powerful early warning system for the movement of P. graminis. This methodology may be useful for monitoring the movement of other plant pathogens dispersed in the atmosphere.